I have a underactive thyroid, a goiter and a positive thyroid antibody test?

What does this mean?? my doctor has told me i have all this but shes not really explained anything, iv been prescribed 50mg thyroxin 10 days ago but im still really tired and cold, im going for a scan on my throat next week. Im a 30 years old woman suffering with all this plus Multiple Sclerosis, i have also just had a baby 4 month ago. can anyone help me and shed some light on my illnesses, whats going on?? Are these all symptoms of something else or am i just really unlucky?